The gas diffusion layer (GDL) is a porous and electrically conductive material located between the catalyst layers and bipolar plates of a polymer electrolyte fuel cell. Its primary function is to provide pathways for (1) reactant gases from the bipolar plate to the catalyst layer, and (2) product water from the catalyst layer to the bipolar plate. The GDL also assists with heat removal and provides mechanical support to the membrane. To further improve both gas and water transport and enhance electrical contact with the catalyst layer, a microporous layer (MPL) is applied to the GDL. In this configuration, the GDL consists of two layers: a macroporous carbon fiber substrate and a microporous composite layer. The MPL is typically composed of carbon black powder and a hydrophobic agent such as PTFE. The MPL prevents the formation of large liquid droplets close to the catalyst layer thereby preventing blockage of the catalyst sites to the incoming reactant gases (JPA_2002-313359). The MPL is typically prepared by mixing carbon black, PTFE and a solvent to create an ink which is applied onto the GDL. The applied layer is then dried at 350° C. to remove the solvent and sinter the PTFE particles.
Current commercial GDLs are made of randomly dispersed carbon fibers resulting in a very porous material with a random and wide pore size distribution. These GDLs have a three dimensional pore structure which has a high tortuosity. During fuel cell operation the porosity is reduced due to compression of the GDL and this leads to low gas permeability and poor gas distribution to the catalyst layer. Water can build up in the compressed pores leading to a phenomenon called GDL flooding. Flooding further prevents gases from reaching the catalyst layer.
Coating of the GDL with PTFE can reduce the build-up of water. The hydrophobic nature of the PTFE results in “fingering” or the formation of narrow water paths to the top surface of the GDL.
Adjacent to the GDL is the bipolar plate. The bipolar plate has a grooved channel and rib structure for reactant gases to flow. It also aids with heat removal from the catalyst layer. The reactant gas diffuses from the channels of the bipolar plate, through the gas diffusion layer and into the catalyst layer. Water generated in the catalyst layer moves towards the channels of the bipolar plate through the GDL. During high current fuel cell operation water tends to accumulate in the GDL areas which are in contact with the rib of the bipolar plate in the so called to “under-rib region”. This water accumulation also leads to GDL flooding.
Current GDL-MPL technologies have low thermal conductivity and electric conductivity because of the high porosity and use of carbon as the core material. This results in higher than desired temperatures in the catalyst layer and membrane which can under certain operating conditions lead to catalyst layer and membrane dry out. Furthermore current GDL technologies have low rigidity such that when the GDLs are compressed during fuel cell operation, there is increased pressure on the GDL areas in contact with the ribs and minimal compression of the GDL areas in the “under-channel region”. The result is poor contact of the GDL with the catalyst layer in certain areas which increases ohmic resistances and decreases fuel cell performance.
To address fuel cell flooding, Goebel (Journal of Power Sources 196 (2011) 7550-7554) reduced the bipolar plate rib width to enhance the water-removal and gas diffusion through the GDL. However reducing the rib width decreases the contact area between the rib and GDL which results in an increase in electronic resistance. Narrow ribs therefore require narrow channels but narrow channels have intrinsic problems of their own, namely higher pressure drop and greater chance of the channels filling up with water leading to channel flooding. The use of a metal GDL allows for the use of a narrow rib and relatively wide channel widths, since if the bipolar plate is also made of metal the contact area between a metal GDL and metal bipolar does not need to be as high as between a carbon GDL and the bipolar plate. The metal GDL therefore allows for the use of narrow ribs without resulting in an increase in electronic resistance. Metal GDLs in general possess significantly lower electronic resistance to carbon GDLs. The electron resistivity of stainless steel for example is 96×10−6 Ωm and that of conventional carbon GDL is 4.7-5.8×10−1 Ωm (http://www.torayca.com/lineup/composites/com_009.html#data).
Pure metal GDLs have pores which are of low tortuosity, probably approaching 1 but slightly increased due to manufacturing tolerances.
Pores in metal GDL's are typically constructed by means of chemical etching. This may be double-sided etching, or single sided etching as illustrated in the cross-sections referred to in FIG. 7:
By making use of chemical etching, the holes in the metal GDL are inherently straight especially for thin plates. Chemical etch is done with acid solution, and the acid is able to etch the metal almost straight. The tolerance is illustrated above. However the holes with this method are not inherently uniform in shape and size. The hole size or shape can be controlled with masking design (typically with a photoresist process).